Sheet turning devices in printing machines are generally known. For example, German patent publication DE 27 56 507 A1 discloses a series-configuration rotary printing machine for face printing and face-and-reverse printing. In this machine, a sheet located on an intermediate drum is gripped at its trailing edge by a pivoting gripper. The gripped edge of the sheet is transported to a second intermediate drum, whereon the gripped edge is the leading edge relative to the direction of cylinder rotation. The pivoting gripper uses mechanical gripping elements to removing and transporting the sheet. Each of these intermediate drums is a sheet-guiding cylinder with a solid lateral surface that is essentially cylindrical, i.e., each cylinder is essentially circular in cross-section. To avoid a collision between the pivoting gripper and one of the drums as the pivoting gripper moves, the intermediate drum has one or more movable segments, with the gripping elements, that move the drum surface radially inwardly at certain times during the swinging movement. The shaft of the pivoting gripper can be moved eccentrically and is synchronized with the rotational movement of the intermediate drum. With the pivoting gripper withdrawn, the printing machine can be operated in face-printing mode.
The device of DE 27 56 507 A1 is disadvantageous in that it requires a high number of parts. For example, a multiplicity of drums must arranged between printing units, and intermediate drum must have the movable lateral-surface segments. Accordingly, this printing machine requires a high outlay for assembly, operation and maintenance. Furthermore, it is necessary to guide the sheet between the drums and the pivoting gripper due to a pronounced curvature of the path along which the sheet is moved. More particularly, smearing would likely result if the pivoting gripper contacted a trailing region of the sheet during a return motion of the pivoting gripper.
A turning apparatus also generally known from German patent publication DE 26 33 183 C2 for a sheet-fed rotary printing machine. This printing machine includes a transfer drum which interacts with a turning drum and a removal device arranged therebetween. The removal device has a gripper arrangement by means of which the sheet, which has been turned on the turning drum, is guided back, with its original trailing edge as the leading edge, to the same transfer drum from which the sheet was sent to the turning drum. The device of DE 26 33 183 C2 is disadvantageous because a turning drum with the associated removal device is required, in addition to the necessary sheet-guiding cylinders, such as impression cylinder and transfer drum.
German patent publication DE 36 02 084 C2 discloses a sheet-transfer drum which is located between printing units of rotary printing machines. The sheet-transfer drum has directing surfaces (flattened sections) between at least two gripper bars. These directing surfaces are designed generally as secants relative to the outer circumference of the drum rotation path. The directing surfaces serve as guide vanes which, during a printing operation, produce a build-up of air that keeps the respective sheet with the printed surface at a distance from the directing surfaces, in an attempt to avoid smearing and double printing.
German patent publication DE-B 12 62 294 discloses a sheet-turning apparatus with a transfer cylinder arranged between the impression cylinders of two printing units of a multicolor rotary sheet-printing machine. During the turning operation, suction grippers, as the first gripper system of the transfer cylinder, attach the sheet by suction at the trailing edge. The first gripper system pivots, on the transfer cylinder, into a clearance and transfers the sheet to a second gripper system, which is likewise arranged on the transfer cylinder. The second gripper system grips and guides the turned sheet, transferring the sheet to a downstream impression cylinder. Disadvantageously, this system requires a considerable number of parts for the transfer cylinder and as a complex and expensive control means for the gripper systems.